Name: boiling point and vapor pressure
Uploaded: 2020-03-26T07:22:27.000Z
Duration: 4 min 25 s
Channel: JoVE

K12 Standards

Georgia Standards of Excellence

9-12

Physical Science

SPS2.

Physical Properties

Boiling Points

Les liquides sont maintenus ensemble par de faibles interactions entre leurs molécules. Pour passer à la phase gazeuse, une molécule doit obtenir suffisamment d’énergie de la chaleur ou d’autres sources pour surmonter ces interactions.
Considérons un récipient fermé d'un liquide pur à température ambiante. Certaines des molécules de la phase liquide à la surface gagnent suffisamment d’énergie pour s’évaporer, et une partie de cette vapeur perd de l’énergie et se condense à nouveau en liquide. Lorsque les taux d’évaporation et de condensation sont les mêmes, le liquide et la vapeur atteignent l’équilibre. À ce stade, l’espace au-dessus du liquide est saturé de vapeur et il n’y a pas de changement net dans la quantité de liquide et de vapeur dans le récipient. La pression exercée par la vapeur est appelée pression de vapeur.
Maintenant, considérez un récipient ouvert de liquide. Ici, la vapeur peut s’échapper, empêchant la zone au-dessus du liquide d’être saturée. Ainsi, l’évaporation pourrait se poursuivre jusqu’à ce qu’il n’y ait plus de liquide. Lorsque la température d’un liquide augmente, le taux d’évaporation augmente également, et par conséquent, la pression de vapeur augmente également. Si suffisamment de chaleur est ajoutée, les molécules plus profondes dans le liquide commencent à se vaporiser, ce que nous voyons comme des bulles de vapeur se formant.
C’est ce qu’on appelle l’ébullition, et cela commence lorsque la pression de vapeur du composé est égale à la pression atmosphérique. La température à laquelle cela se produit est le point d’ébullition. Étant donné que chaque substance pure a des types et des forces différents d’interactions intermoléculaires, différentes substances ont des points d’ébullition différents.
Voyons maintenant comment les interactions intermoléculaires influencent le point d'ébullition. De nombreuses interactions intermoléculaires impliquent des zones à densité électronique inégale appelées dipôles. Toutes les molécules ont de brèves fluctuations dans leur distribution d’électrons, de sorte que même les molécules non polaires peuvent temporairement avoir des dipôles. En réponse, des dipôles opposés se forment dans les molécules voisines.
Les forces d’attraction entre les dipôles temporaires et induits sont appelées forces de dispersion de Londres, et ce sont les interactions dominantes entre les molécules non polaires comme les hydrocarbures. Les forces de dispersion de London sont plus fortes entre les grosses molécules avec de nombreux électrons, car ces molécules retiennent faiblement leurs électrons, ce qui permet aux molécules voisines d’affecter plus facilement leur distribution d’électrons.
Par exemple, un alcane non ramifié de poids moléculaire élevé a un point d’ébullition plus élevé qu’un alcane non ramifié de poids moléculaire inférieur. De plus, un alcane non ramifié a un point d’ébullition plus élevé qu’un alcane ramifié de poids moléculaire comparable en raison de sa surface plus élevée.
Les molécules à liaisons polaires ont des dipôles permanents, permettant des interactions régulières dipôle-dipôle entre elles. Les interactions dipôle-dipôle sont plus fortes que les forces de dispersion de Londres, de sorte qu’il faut généralement plus d’énergie thermique pour faire bouillir des liquides polaires que des liquides non polaires. Par exemple, le point d’ébullition d’un chloropentane, une molécule polaire, est de 108 °C. L’hexane, qui a une surface similaire mais n’a pas de dipôle, a un point d’ébullition plus bas.
Les molécules avec à la fois un hydrogène lié de manière covalente à un atome plus électronégatif et un atome de retrait d’électrons avec une paire non liante d’électrons sont capables de se lier à l’hydrogène. Les liaisons hydrogène sont plus fortes que les interactions dipôle-dipôle et les forces de dispersion de Londres, il faut donc encore plus d’énergie thermique pour les surmonter. Par exemple, le propane et l’éthanol ont des poids moléculaires et des surfaces similaires. Mais le point d'ébullition du propane est beaucoup plus bas que celui de l'éthanol, car les molécules d'éthanol peuvent former des liaisons hydrogène entre elles, contrairement au propane.
Dans ce laboratoire, vous utiliserez la méthode capillaire pour déterminer le point d’ébullition de deux solvants organiques.

points d’ébullition

Détermination du point d’ébullition
Semblable au point de fusion, le point d’ébullition est une propriété physique. Si l’échantillon est un composé pur, le point d’ébullition peut être utilisé pour déterminer l’identité du composé. En fin de compte, il est difficile de déterminer expérimentalement le point d’ébullition exact. Comme les points de fusion, les points d’ébullition expérimentaux sont donnés sous forme de plage et varient de quelques degrés par rapport à la valeur réelle de la littérature.
Pression de vapeur
Pour comprendre pourquoi un solvant bout, ce qui se caractérise par le bouillonnement familier de la solution, il est important de comprendre la dynamique entre les phases liquide et gazeuse. Considérons un composé liquide pur dans un récipient scellé. Certaines des molécules à la surface du liquide auront suffisamment d’énergie pour surmonter les forces intermoléculaires et entrer dans la phase gazeuse. Cependant, les molécules en phase gazeuse peuvent également perdre de l’énergie et se condenser à nouveau en un liquide. Par conséquent, il existe deux processus concurrents dans ce système : l’évaporation et la condensation.
Lorsque le taux d’évaporation est égal au taux de condensation, le système a atteint un état d’équilibre. Cela signifie que pour chaque molécule qui entre dans la phase gazeuse, une autre se condense en phase liquide, et il n’y a pas de gain ou de perte net de la quantité de liquide ou de gaz dans le récipient. Une fois l’équilibre établi, la pression exercée par la vapeur au-dessus du liquide est appelée pression de vapeur. La tendance d’un liquide à se vaporiser s’appelle sa volatilité. Un liquide plus volatil a une pression de vapeur plus élevée, tandis qu’un liquide moins volatil a une pression de vapeur plus faible.
La pression de vapeur varie en fonction de la température. Si vous augmentez la température de la solution, plus de molécules ont suffisamment d’énergie pour s’échapper de la phase liquide, et donc la pression de vapeur augmente. En fin de compte, si suffisamment de chaleur est appliquée, les molécules qui ne sont pas à l’interface entre le liquide et le gaz passeront en phase gazeuse et formeront les bulles familières que nous associons à l’ébullition.
Le point d’ébullition d’un liquide est atteint lorsque la pression de vapeur totale du liquide est équivalente à la pression atmosphérique. La température à laquelle cela se produit est appelée point d’ébullition. À des altitudes plus élevées, et donc à une pression atmosphérique plus basse, un liquide bouillira à une température plus basse, car moins de chaleur est nécessaire pour augmenter la pression de vapeur à la pression atmosphérique. De plus, la volatilité, ou la capacité d’un solvant à se vaporiser, a également un impact sur la pression de vapeur. Les solvants à haute volatilité ont une pression de vapeur plus élevée que les solvants à faible volatilité.
Facteurs qui ont un impact sur le point d’ébullition
La similitude entre les points de fusion et les points d’ébullition signifie que les mêmes facteurs qui ont un impact sur le point de fusion d’un composé auront également un impact sur le point d’ébullition. Par conséquent, la force et les types de forces intermoléculaires que l’on trouve dans le composé liquide affecteront le point d’ébullition. Rappelons qu’il existe trois types de forces moléculaires : les liaisons hydrogène, les interactions dipôle-dipôle et les forces de dispersion de Londres. Chacun d’entre eux a des forces d’attraction différentes et nécessite différentes quantités d’énergie pour être surmonté. Les composés qui peuvent se lier à l’hydrogène auront des points d’ébullition plus élevés que les composés qui ne peuvent interagir que par les forces de dispersion de Londres. Une considération supplémentaire pour les points d’ébullition implique la pression de vapeur et la volatilité du composé. En règle générale, plus un composé est volatil, plus son point d’ébullition est bas.
La méthode capillaire pour déterminer le point d’ébullition
Une méthode simple pour déterminer le point d’ébullition d’un composé organique consiste à utiliser la méthode capillaire. Dans cette configuration, un tube capillaire en verre vide est inversé dans un récipient du composé pur en phase liquide. Au fur et à mesure que le liquide est chauffé, la pression de vapeur de l’échantillon augmente et la vapeur gazeuse commence à pénétrer dans le tube capillaire en verre. Cela force l’air emprisonné à l’intérieur et entraîne l’émergence de bulles du fond du tube capillaire. À ce stade, on laisse refroidir le liquide. Une fois que la pression de vapeur de l’échantillon est la même que la pression atmosphérique à l’intérieur du tube capillaire en verre, le liquide commencera à pénétrer dans le tube. La température de la solution lorsque ce phénomène se produit est le point d’ébullition du composé liquide.

Points d’ébullition

Détermination du point d’ébullition
Semblable au point de fusion, le point d’ébullition est une propriété physique. Si l’échantillon est un composé pur, alors l’ébullition ...

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boiling point and vapor pressure

Boiling point and vapor pressure are key physical properties of a liquid. For a pure compound, the boiling point can help identify the substance. In experiments, the boiling point is usually recorded as a range because exact values are hard to measure and may differ by a few degrees from the literature value.

Vapor pressure explains why a liquid boils. In a sealed container, some molecules at the liquid surface gain enough energy to escape into the gas phase. At the same time, some gas molecules lose energy and condense back into the liquid. When evaporation and condensation happen at the same rate, the system is at equilibrium. The pressure from the vapor above the liquid is the vapor pressure.

Volatility is the tendency of a liquid to vaporize. A more volatile liquid has a higher vapor pressure, while a less volatile liquid has a lower vapor pressure. Vapor pressure also changes with temperature. As temperature rises, more molecules can escape the liquid, so vapor pressure increases. With enough heat, molecules inside the liquid also change to gas and form the bubbles seen during boiling.

A liquid boils when its total vapor pressure matches atmospheric pressure. That temperature is the boiling point. Because atmospheric pressure changes with elevation, a liquid boils at a lower temperature at higher elevations. Less atmospheric pressure means less heat is needed for the vapor pressure to reach that level.

Intermolecular forces also affect boiling point. These forces include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. They differ in strength, so different amounts of energy are needed to overcome them. Compounds that can hydrogen bond usually have higher boiling points than compounds that rely only on London dispersion forces. Volatility matters too. In general, more volatile compounds have lower boiling points.

One simple way to measure a boiling point is the capillary method. An empty glass capillary tube is placed upside down in the liquid sample. As the sample is heated, its vapor pressure rises and vapor enters the tube. This pushes out trapped air and causes bubbles to appear from the bottom of the capillary tube. The sample is then allowed to cool. When liquid begins to rise into the tube, the vapor pressure inside the tube matches the atmospheric pressure. The temperature at that moment is the boiling point of the compound.

Boiling Point and Vapor Pressure

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Boiling Point and Vapor Pressure

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